@JohnJohn25 With Deco mesh running in AP mode you will have to set custom DNS on router to which Main Deco is connected.

@Antoine_B As I thought. It appears to be common issue among long term owners of Deco M9 Plus. For solution, see: https://community.tp-link.com/en/home/forum/topic/671334?replyId=1369784

@Antoine_B I saw this error message when not all Deco units in Deco mesh were updated to the version of firmware that supports Connection Preference feature. Make screenshot of Update Deco page from...

@Rich24 Check this link to see if your Deco M4 can be upgraded to firmware that supports connection preference: https://community.tp-link.com/en/home/forum/topic/651248 This is the feature you need...

@Renard Here is the summary of changes to your home network I am going to recommend: The plan is to take Main Deco from cottage, bring it to house and connect to Ethernet cable that comes from the...

@Renard I am going to start with an assessment of what you have, and then will provide you with the plan that you should be able to execute now, and without need for getting new or additional...